790 REGINALD ALDWORTH DALY 
it may be asked, why does the porphyritic granite not show more 
variations of mineral content? To this question we have no 
conclusive answer. It is possible that the rock which now fills 
the areas of the porphyritic granite was in general not ata tem- 
perature high enough to cause vigorous melting up of the walls, 
and that the great spaces in the earth-crust now filled by the 
granite were opened during the passage upward of earlier and 
hotter parts of the same magna. However this may be, the dif- 
ficulty remains just as great for any theory of a metamorphic 
origin for the granite. It is impossible to believe that a rock 
with such continuity of like characters should have resulted 
from the alteration of the stratified or schistose rocks in the 
accompanying terranes of New Hampshire. Although the deter- 
minations of relative age among these terranes are as yet neces- 
sarily imperfect, we know that the porphyritic granite is in 
contact with rocks of many different horizons and of very varia- 
ble composition. From this fact, it seems reasonable to con- 
clude that the porphyritic granite is an exotic eruptive, finding 
its source of supply elsewhere than in any metamorphic center 
in immediate connection with the encircling schists. McMahon 
lays considerable stress on this idea in his argument for an 
eruptive origin of the Himalayan granites.‘ Lawson, in his 
study of the Laurentian gneisses, and Barlow, in a similar prob- 
lem among the ancient rocks north of Lake Huron,’ refer their 
” 
‘irruptive ’ masses to a fusion of the granitic floor on which the 
post-Laurentian rocks were laid. The New Hampshire rock is 
thus, in respect to its origin, more closely allied to the Hima- 
layan granite than to the gneiss of the Canadian Laurentian. 
Coarse veins cutting porphyritic granite— McMahon and others 
describe plutonic eruptives intersected by dikes and veins of 
very similar material to that of which their hosts are composed. 
Besides the usual evidences of aneruptive origin for the Dal- 
housie gneissose granite of northern India, McMahon adds a 
criterion which is certainly without the weight of its associates, 
™ Geol. Mag., 1887, p. 216; Rec. Geol. Sury. India, XVIII, 1885, p. 106. 
2 Bull. Geol. Soc. Am., IV, 1893, p. 331. 
